Apparatus for coating a web

ABSTRACT

An apparatus for coating a web of indeterminate length has a coating element comprising a liquid deflector member for diverting liquid away from a coating surface. The liquid deflector member is arranged beneath a blade member that removes excess coating liquid from the coating surface. Excess coating liquid follows a path away from the coating surface and down the liquid deflector member thereby avoiding contamination of the coating surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is related to U.S. application Ser. No.(Docket 78832), filed ______, by Ramasubramaniam Hanumanthu, et al., andentitled, “Element For Deflecting Excess Liquid From A Coating Surface.”

FIELD OF THE INVENTION

[0002] The invention relates generally to the field of roller/gravurecoating. More particularly, the invention concerns a coating apparatusthat meters a film of liquid coating solution from a coating surface orroller and then diverts it away, thereby preventing contamination of thecoating surface.

BACKGROUND OF THE INVENTION

[0003] In conventional roller/gravure coating processes (as described,for example, in U.S. Pat. No. 4,373,443, Feb. 15, 1983, by Matalia etal., entitled, “Method Of High Viscosity Inking In Rotary NewspaperPresses” where a gravure cylinder provides ink in newspaper presses), aliquid coating composition is directed to the surface of a coatingapplicator roll 1 by one of several suitable means including rotating(denoted by arrow) the applicator roll 1 through a reservoir 2 of liquid3, as illustrated in FIG. 1. The surface of the coating applicator roll1 may have a smooth finish or it may be engraved with cells/grooves 5 ofprescribed volume. Often, the layer of liquid 3 picked up by theapplicator roll 1 from the reservoir 2 is subsequently metered to athinner film using a doctor blade 4. In gravure coating, for example,the doctor blade 4 removes all the applied liquid except that which ispresent in the engraved cells 5 formed in the gravure cylinder 1.Alternatively, the steps of wetting (filling) and doctoring may also becombined as described in U.S. Pat. No. 4,158,333, Jun. 19, 1979, byNavi, titled, “Inking Baffle For Rotary Newspaper Presses.” After thedoctoring step, the liquid remaining on the surface of a smooth coatingapplicator roll or that remaining in the cells 5 of an engraved coatingapplicator roll is transferred to a moving web 6 by impressing themoving web 6 between the applicator roll 1 and a soft backer orimpression roll 7. In FIG. 1, the web 6 is shown to be moving in thesame direction as the surface of the coating applicator roll 1 at thepoint of contact between the two, but in roller/gravure coatingpractice, the web may be conveyed in the opposite direction as well. Thethickness of coating transferred to the moving web 6 is generally aknown fraction of the thickness of liquid film retained on the surfaceof a smooth coating applicator roll downstream of the doctoring step or,alternatively, it is a known fraction of the volume of the engravedcells 5 per unit surface area of an engraved coating applicator roll 1.

[0004] Depicted in FIGS. 2a and 2 b , a shortcoming of existingroller/gravure coating processes is that when excess liquid 8 removed bythe doctor blade 4 falls back on the surface of the coating applicatorroll 1, it is carried back up to the “bank” of coating liquid 9 that isaccumulated between the moving coating applicator roll 1 surface and thestationary doctor blade 4. Since the excess liquid 8 falls back on andcontacts the surface of the coating applicator roll 1 in a turbulent andrandom manner, this renders the bank of coating liquid 9 uneven in thecross-web direction. The unevenness of the bank of coating liquid 9 inturn causes a coating defect in the form of streaks and bands 10, asexemplified in FIG. 3. The defect is especially prominent in particulatecoating dispersions (as opposed to solutions).

[0005] An analysis of the nature of the flow of metered liquid 3 behindthe doctor blade 4 reveals that at low coating applicator roll 1 surfacespeeds the liquid 3 simply runs back down the surface of the coatingapplicator roll 1 in a laminar fashion (see flow lines 11 in FIG. 4a).However, as speed of the coating applicator roll 1 is raised, a point isreached when the metered liquid 3 separates from the surface of thecoating applicator roll 1 and flows (see flow lines 12 in FIG. 4b)generally along the underside 13 of doctor blade 4 and away from thesurface of the applicator roll 1.

[0006] Moreover, at some point further downstream of the contact point14 between the doctor blade 4 and the coating applicator roll 1, thedeflected liquid loses its momentum and therefore separates from theunderside surface 13 of the doctor blade 4 and falls or flows verticallydownwards under the influence of gravity (refer to FIG. 4b).

[0007] Presently the defect can be avoided in one of several ways. Oneway known to avoid this defect is to maintain the coating speed belowthe speed of transition from “runback” flow to “deflected” flow.Experimental observations indicate that the speed of transition betweenrunback flow (FIG. 4a) and deflected flow (FIG. 4b) depends on operatingparameters—viscosity and surface tension of liquid; tangent anglebetween doctor blade 4 and surface of the coating applicator roll 1;thickness of the incoming film of liquid; radius of coating applicatorroll 1; etc. Here, runback flow is defined as the case where liquidremoved by the doctor blade 4 runs back down the surface of the coatingapplicator roll 1. Deflected flow is where the excess liquid 8 meteredby the doctor blade 4 travels away from the surface of the coatingapplicator roll 1, along the underside 13 of the doctor blade 4, up to apoint where it loses its momentum, and then further separates from theunderside 13 of the doctor blade 4 surface, and drops vertically underthe influence of gravity.

[0008] Unfortunately, under normal operating/manufacturing conditions,the speed of transition from runback to deflected flow is too low for itto be a practicable production speed.

[0009] Referring to FIGS. 5a and 5 b , another known way to avoid thedefect is to locate the contact point or tip 14 of the doctor blade 4 atapplication points on the cylindrical coating applicator roll 1 surfacethat are far from top-dead-center 19. Then, especially in the case ofsmall diameter cylinders, i.e., typically diameters less than about 5inches, the deflected excess liquid 8 in all likelihood will not flowback to the cylindrical coating applicator roll 1 surface on its waydown (refer to FIG. 5b). But at application points close totop-dead-center 19, and with large diameter coating applicator rolls 1,the excess liquid 8 will tend to flow back to the surface of the coatingapplicator roll (FIG. 5a).

[0010] Unfortunately, the location of the contact point or tip 14 of thedoctor blade 4, relative to top-dead-center 19 cannot be changedarbitrarily. For instance, to minimize evaporation of coating liquid 3from the surface of the coating applicator roll 1 in the region betweenthe contact point or tip 14 of the doctor blade 4 and top-dead-center19, it may be necessary to narrowly fix the distance of the contactpoint or tip 14 of the doctor blade 4 from top-dead-center 19.Similarly, the diameter of the coating applicator roll 1 may also haveto be narrowly fixed. This is true, for instance, in the coating ofdiscrete patches or patterns using gravure coating, wherein the ratio ofgravure cylinder circumference to engraved patch/pattern length has tobe maintained constant.

[0011] While there are no known prior art attempts to solve Applicants'specific problem of diverting coating liquid from the surface of acoating applicator roll having an excess quantity of liquid thereon,U.S. Pat. No. 5,755,883, May 26, 1998, by Kinose et al., titled, “RollCoating Device For Forming A Thin Film Of Uniform Thickness” discloses aroll coater having a blade scraper for scraping coating liquid from ametal roll and a tray positioned beneath the nip for catching thescraped liquid. This device provides only for preventing fluid fromcontacting coating elements beneath the nip and does not protect theroll from which the liquid was deposited from receiving excess liquid.

[0012] An attempt to use a similar tray in a location between theunderside 13 of the doctor blade 4 and the surface of the coatingapplicator roll 1 (refer to FIG. 6) was not successful because there isvery little room available there. Indeed the deflected excess liquid 8separates from the underside 13 of the doctor blade 4 so quickly thatthe lip 20 of the tray 21 would have to be within 0.32 cm (0.125 in)from the underside surface 13 of the doctor blade 4, and the applicatorroll 1 surface. Such tight gaps are not favored in manufacturingenvironments.

[0013] Yet another scheme to prevent the defect involves the creation ofa narrow passageway 22 between the coating applicator roll 1 surface andan element 23. The coating liquid 3 effectively “floods” the passageway22 and in this manner defects that persist far upstream of the contactpoint or tip 14 of doctor blade 4 are forced to damp out before theyreach the contact point or tip 14 of doctor blade 14. In other words,the pressure in the “bank” of coating liquid 9 accumulated between themoving coating applicator roll 1 surface and the stationary doctor blade4 stays even across the width of the web 6, at least in the vicinity ofthe doctor blade tip 14. However, the drawback of this approach was thatto effectively flood the passageway 22 under all operating conditions,the element 23 had to be maintained at gaps less than 0.2 cm (0.08 in)from the coating applicator roll 1 surface. Again, such narrow gaps arenot favored in the manufacturing environment.

[0014] Finally, the problem may be inherently solved by using combinedfeed/blading units, such as the reverse doctor pond feed (U.S. Pat. No.4,158,333). There, the trailing blade at the exit of the reservoir keepsthe excess fluid within the reservoir, and hence there is no occasionfor deflection (“deflection” is illustrated in FIG. 4b). However, in thepresent application, reverse doctor pond feed is not practicable.

[0015] Therefore, there persists a need for a roller/gravure coatingprocess in which excess coating liquid material removed by a doctorblade is diverted away from the surface of the coating applicator rollthereby avoiding contamination of the applicator roll surface.

SUMMARY OF THE INVENTION

[0016] It is, therefore, an object of the invention to provide aroller/gravure coating apparatus having a liquid metering/divertingelement for metering a film of liquid material from the surface of acoating applicator roll and then diverting excess liquid material awayfrom the surface of the coating applicator roll.

[0017] An important feature of the invention is a liquid deflectormember arranged proximate to the surface of the coating applicator rolland a metering member for diverting excess liquid away from the coatingapplicator roll surface.

[0018] To solve this and other objects of the invention, there isprovided an apparatus for coating a web of indeterminate length,comprising a source of coating composition; an engraved cylinder atleast partially in fluid contact with the source of coating composition.The engraved cylinder includes a plurality of cells for collectingcoating composition therein and then transfers the coating compositionto the web of indeterminate length. An impression cylinder is inrotating contact with the engraved cylinder, which thereby forms a webtransfer path therebetween. The web of indeterminate length is advancedthrough the web transfer path so that coating composition in pluralityof cells transfers to the web of indeterminate length forming an appliedcoat of coating composition on the web of indeterminate length. Theapparatus also comprises a coating element for doctoring the appliedcoat of coating composition on the web of indeterminate length to afinished coat and then diverting any excess coating composition awayfrom said engraved cylinder.

[0019] It is an advantageous effect of the invention that the liquiddeflector member is versatile, cost effective to manufacture, simple toinstall and operate and can function with minimum variability ofsettings over a wide range of manufacturing operating conditions

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above and other objects, features, and advantages of thepresent invention will become more apparent when taken in conjunctionwith the following description and drawings wherein identical referencenumerals have been used, where possible, to designate identical featuresthat are common to the figures, and wherein:

[0021]FIG. 1 is a front elevation view of a prior art roller/gravurecoating process;

[0022]FIG. 2a is a schematic illustration of a partial cross-sectionalside view of a prior art roller/gravure coating process illustratingdoctored sheet of coating liquid flowing downwardly onto the surface ofa coating applicator roll;

[0023]FIG. 2b is a scanned photographic image of a partialcross-sectional side view of a prior art roller/gravure coating processillustrating doctored sheet of coating liquid flowing downwardly ontothe surface of a coating applicator roll;

[0024]FIG. 3 is a scanned image of a coating sample illustrating defectsin the form of streaks and bands of a prior art roller/gravure coatingprocess;

[0025]FIG. 4a is a schematic of a roller/gravure coating processillustrating flow of excess coating liquid running back down the surfaceof a coating applicator roll;

[0026]FIG. 4b is a schematic of a roller/gravure coating processillustrating deflected flow of excess coating liquid along the undersideof a doctor blade member;

[0027]FIG. 4c is a scanned image of photographic snapshots depicting thetransition of flow behind the blade from “runback” to “deflected” modes.The top and bottom pictures images in this column are the counterpartsof the schematic illustrations in FIGS. 4a and 4 b, respectively;

[0028]FIG. 5a is a schematic of a prior art roller/gravure coatingprocess illustrating deflected sheet of coating liquid separating fromunderside of doctor blade and flowing downwardly onto the surface of acoating applicator roll;

[0029]FIG. 5b is a schematic of a prior art roller/gravure coatingprocess illustrating deflected sheet of coating liquid separated fromthe doctor blade and the surface of a coating applicator roll;

[0030]FIG. 6 is a schematic of a prior art element to catch thedeflected sheet of liquid after separation from the doctor blade;

[0031]FIG. 7 is a schematic of another prior art element to flood thepassageway between the surface of a coating applicator roll and saidelement in an attempt to maintain an even bank of coating liquid at thetip of the blade;

[0032]FIG. 8a is a schematic of the element of the inventionillustrating orientation with respect to the surface of the coatingapplicator roll and metering doctor blade;

[0033]FIG. 8b is a scanned image of an application of the invention;

[0034]FIG. 9 is a schematic of the element of the invention illustratingan unfavorable orientation of liquid deflector member; and, FIGS. 10a ,10 b, and 11 are schematics of the element of the invention illustratingalternative embodiments.

DETAILED DESCRIPTION OF THE INVENTION

[0035] Turning now to the drawings, and in particular to FIGS. 8a-10,there is illustrated the coating element 25 according to the principlesof the invention. According to FIGS. 8a and 8 b , coating element 25removes excess liquid (l) from the surface 27 of a coating applicator,such as a roll 24, and then diverts the excess liquid (l) away from thesurface 27. Importantly, coating element 25 has a doctor blade member 26and a liquid deflector member 28 structurally disposed in a supportmember 30.

[0036] Referring to FIGS. 8a-11, doctor blade member 26, generally hasan active end 32 extending from the support member 30 for engaging andremoving excess liquid (l) from the surface 27 of coating applicator orroll 24. Support member 30 is used principally to manipulate and fix theorientation of the active end 32 relative to the surface 27 of thecoating applicator or roll 24. Thus, for most efficient operation,active end 32 of doctor blade member 26, and more particularly,underside 34, is arranged preferably at a predetermined angle θ_(t) withthe surface 27 of the coating applicator or roll 24. The inventors havedetermined that a preferred range of predetermined angle θ_(t) isbetween about 50-60 degrees. Skilled artisans will appreciate that theactive end 32 of the doctor blade member 26 contacts the surface 27 ofthe coating applicator or roll 24 at some well defined point P so thatexcess coating liquid (l) can be effectively removed from the surface27.

[0037] Referring to FIGS. 8a-11, liquid deflector member 28 has anactive face 36 (if properly oriented) that diverts excess coating liquid(l) away from the surface 27 of the coating applicator or roll 24. Thus,excess coating liquid (l) doctored from the surface 27 of coatingapplicator or roll 24 flows along the underside 34 of active doctorblade member 26 and then along active face 36 of liquid deflector member28 away from surface 27. Active face 36 is positioned proximate to boththe active end 32 of the doctor blade member 26 and the surface 27 ofthe coating applicator or roll 24. The underside 34 of doctor blademember 26 extends from the contact point P to apex 38 by a predeterminedclearance (d), described further below. Apex 38 is a point on theunderside 34 of blade member 26 that intersects the active face 36 ofthe liquid deflector member 28. Further, active face 36 of liquiddeflector member 28 is arranged at a predetermined angle θ_(s) to theunderside 34 of the active end 32 of doctor blade member 26. In thepreferred embodiment, active face 36 of liquid deflector member 28 isgenerally planar (FIG. 8a). Alternately, active face 36 may be generallycontoured from a point near apex 38 either away (FIG. 10a) from thesurface 27 of coating applicator or roll 24 or towards (FIG. 10b) thesurface 27 of coating applicator or roll 24. Each of theseconfigurations has proven effective in diverting excess liquid (l) awayfrom surface 27.

[0038] Referring again to FIG. 8a , the underside 34 of doctor blademember 26 preferably makes a generally obtuse angle with the adjoiningactive face 36 of the liquid deflector member 28. Thus, excess liquid(l) will follow a generally obtuse angular path from the underside 34 ofthe doctor blade member 26 along the active face 36 of the liquiddeflector member 28.

[0039] Referring now to FIG. 11, alternatively, the underside 34 ofdoctor blade member 26 may form a generally arcuate path with the activeface 36 of the liquid deflector member 28 along which excess liquid (l)flows.

[0040] Referring again to FIGS. 8a and 8 b , liquid deflector member 28is adjustably fixed to support member 30 with active face 36 positionedclose enough to the contact point P that it “captures” the deflectedliquid (l) flowing on the underside 34 of doctor blade member 26. Thepositioning is important because the deflected liquid (l) could verywell lose its momentum and then divert downwardly under the influence ofgravity towards surface 27 of the coating applicator or roll 24.

[0041] Liquid deflector member 28, preferably made of a rigid metal orplastic, may be structurally affixed to support member 30 in severalways with virtually the same results, including bolting, screwing,riveting, welding, or clamping.

[0042] Referring again to FIGS. 8a and 8 b , there are several importantoperating constraints on the design of the liquid deflector member 28.According to FIG. 8a , the angle θ_(s) that the liquid deflector member28 makes with the underside 34 of the doctor blade member 26 is optimumwhen the active face 36 of the deflector member 28 is near normal to thedoctor blade member 26. However, in this configuration, there is a highrisk that a liquid deflector member 28 having a rather long length mightinterfere with the rotating surface 27 of coating applicator or roll 24.Consequently, our experience indicates that a preferred angle θ_(s) isone that is equal to the tangent angle θ_(t). When θ_(s) is less thanθ_(t), full advantage is not taken of the assist that gravity providesto the flow of deflected liquid (l) down the active face 36 of deflectormember 28 away from the surface 27 of coating applicator or roll 24. Onthe other hand, if 05 is much larger than θ_(t), there is a rather highrisk that the bottom edge 40 of the liquid deflector member 30 mightinterfere with the surface 27 of the coating applicator or roll 24further upstream of the doctor blade member 26 (refer to FIG. 9).

[0043] Referring to again FIG. 8a , as indicated, it is also importantthat the underside 34 of doctor blade member 26 have a predeterminedclearance (d), i.e., distance between the apex 38 and the contact P. Fora given inclination, θ_(h) of blade member 26 above the horizontalplane, this optimum predetermined clearance (d) depends on the flow rateof deflected liquid (l) (per unit width of coating), q; viscosity ofcoating liquid, μ; density of coating liquid, ρ; and gravitationalacceleration, g: clearance ∝${\left( \frac{q^{2}}{g} \right)^{1/3} \cdot f},$

[0044] f, where f is a monotonically increasing function of theReynolds' Number (Re), given by${Re} \equiv {\frac{q\quad \rho}{\quad \mu}.}$

[0045] In the preferred embodiment, an effective clearance (d) is one inthe range of about 0.64 cm (0.25 in) to about 1.9 cm (0.75 in).

[0046] The invention has been described with reference to a preferredembodiment. However, it will be appreciated that variations andmodifications can be effected by a person of ordinary skill in the artwithout departing from the scope of the invention. PARTS LIST: l excesscoating liquid P point of contact 1 coating applicator roll 2 reservoiror pan 3 liquid or coating liquid 4 doctor blade 5 .engravedcells/grooves 6 web 7 soft backer or impression roll 8 excess liquid 9bank of coating liquid 10 streaks and bands 11 flow line 12 flow line 13underside of doctor blade 4 14 contact point or tip of doctor blade 4 19top-dead-center of cylindrical surface of coating applicator roll 1 20lip of tray 21 21 tray 22 narrow passageway 23 element 24 coatingapplicator or roll 25 coating element 26 doctor blade member 27 surfaceof coating applicator 28 liquid deflector member 30 support member ofcoating element 25 32 active end of doctor blade member 26 34 undersideof doctor blade member 26 36 active face of liquid deflector member 2838 apex 40 bottom edge of liquid deflector member 28

What is claimed is:
 1. Apparatus for coating a web of indeterminatelength, comprising: a source of coating composition; an engravedcylinder at least partially in fluid contact with said source of coatingcomposition, said engraved cylinder having a plurality of cells forcollecting coating composition therein and then transferring saidcoating composition to said web of indeterminate length; an impressioncylinder in rotating contact with said engraved cylinder, saidimpression cylinder and said engraved cylinder forming a web transferpath therebetween; means for advancing said web of indeterminate lengththrough said web transfer path so that coating composition in saidplurality of cells transfers to said web of indeterminate length formingan applied coat of coating composition on said web of indeterminatelength; and, a coating element for doctoring said applied coat ofcoating composition on said web of indeterminate length to a finishedcoat and then diverting any excess coating composition away from saidengraved cylinder.
 2. The apparatus recited in claim 1 wherein saidcoating element comprises: an element support member; a blade memberstructurally associated with said element support member, said blademember having an active end extending from said element support memberfor engaging said engraved cylinder, said blade member being arranged ata predetermined angle θ_(t) with said engraved cylinder and having apoint of contact therewith; and a liquid deflector member structurallyassociated with said element support member and said blade member, saidliquid deflector member having an active face arranged proximate to saidactive end of said blade member and to said engraved cylinder.
 3. Theapparatus recited in claim 1 wherein said liquid deflector memberextends downwardly away from said blade member such that excess liquidremoved by said blade member from said engraved cylinder flows along apath defined by said blade member and said liquid deflector member. 4.The apparatus recited in claim 2 wherein said active face of said liquiddeflector member is inclined at a predetermined angle θ_(s) relative tosaid active end of said blade member.
 5. The apparatus recited in claim4 wherein said active face of said liquid deflector member is generallycontoured away from said engraved cylinder.
 6. The apparatus recited inclaim 4 wherein said active face of said liquid deflector member isgenerally contoured towards said engraved cylinder.
 7. The apparatusrecited in claim 4 wherein said active face of said liquid deflectormember is generally planar.
 8. The apparatus recited in claim 4 whereinsaid predetermined angle θ_(s) is equal to or greater than saidpredetermined angle θ_(t).
 9. The apparatus recited in claim 2 whereinsaid blade member has an underside that extends from said point ofcontact to said active face of said liquid deflector member, saidunderside defining a predetermined clearance.
 10. The apparatus recitedin claim 9 wherein said predetermined clearance for a predeterminedinclination θ_(h), is predicted by the relationship: clearance ∝${\left( \frac{q^{2}}{g} \right)^{1/3} \cdot f},$

f, wherein: f is a monotonically increasing function of the Reynolds'Number (Re), given by ${{Re} \equiv \frac{q\quad \rho}{\quad \mu}};$

q is flow rate of deflected liquid (per unit width of coating); μ isviscosity of coating liquid; ρ is density of coating liquid; and g isacceleration due to gravity.
 11. The apparatus recited in claim 9wherein said predetermined clearance is in the range between about 0.64cm (0.25 in) and 1.9 cm (0.75 in).
 12. The apparatus recited in claim 9wherein a generally arcuate path is formed between said underside ofsaid blade member and said active face of said liquid deflector member.13. The apparatus recited in claim 9 wherein a generally obtuse angularpath is formed between said underside of said blade member and saidactive face of said liquid deflector member.